The outpouring of lava

CALDERA DEVELOPMENT. Schematic diagrams showing idealized stages in the development of the Yellowstone caldera 600,000 years ago. The scales shown in Diagram A are approximately the size of the features in Yellowstone. Although only one magma chamber is pictured in the diagrams, two chambers were involved in the Yellowstone eruption. (Based on information supplied by R. L. Christiansen and H. R. Blank, Jr.) (Fig. 23)A, A large magma chamber formed deep within the earth, and the molten rock began to force its way slowly toward the surface. As it pushed upward, it arched the overlying rocks into a broad dome. The arching produced a series of concentric fractures, or a ring fracture zone, around the crest of the dome. The fractures extended downward toward the top of the magma chamber.

A, A large magma chamber formed deep within the earth, and the molten rock began to force its way slowly toward the surface. As it pushed upward, it arched the overlying rocks into a broad dome. The arching produced a series of concentric fractures, or a ring fracture zone, around the crest of the dome. The fractures extended downward toward the top of the magma chamber.

B, The ring fractures eventually tapped the magma chamber, the uppermost part of which contained a high proportion of dissolved gases. With the sudden release of pressure, tremendous amounts of hot gases and molten rock were erupted almost instantly. The liquid solidified into pumice, ash, and dust as it was blown out. Some of the dust and ash was blown high into the air and carried along by the wind, but much of the debris moved outward across the landscape as vast ash flows, covering thousands of square miles very rapidly.

C, The area overlying the blown-out part of the magma chamber collapsed to form a gigantic caldera. The collapse took place mostly along normal faults that developed from the fractures in the ring fracture zone. The depth of the collapse was probably several thousand feet.

D, Renewed rise of molten rock domed the caldera floor above the magma chamber. A series of rhyolite lava flows poured out through fractures in the surrounding ring fracture zone and spread across the caldera floor.

ORIGINAL EXTENT OF THE YELLOWSTONE TUFF (ash-flow tuff) that covered most of Yellowstone National Park about 600,000 years ago. The tuff was erupted explosively from the ring fracture zones of the Yellowstone caldera. The outline of the caldera is shown by the dashed line. (Based on information supplied by R. L. Christiansen and H. R. Blank, Jr.) (Fig. 24)

YELLOWSTONE TUFF AT GOLDEN GATE. The rocks consist of layered ash-flow tuff; the height of the cliff is about 200 feet. (Fig. 25)

Closeup B shows typical characteristics of the tuff in most outcrop areas. Of the light-colored materials, the larger masses are compressed pumice fragments and the smaller masses are pumice, feldspar, and quartz. The dark grains are chiefly magnetite and pyroxene. Closeup A is of a coarse-grained specimen from Tuff Cliff. The large fragments are mostly crystallized pumice, and the light-colored matrix is composed of very fine particles of volcanic ash and dust.

GEOLOGIC CROSS SECTION showing generalized relationships along the north edge of the Yellowstone caldera in the Mount Washburn-Canyon area (line of section labeled D-D′ onpl. 1). The caldera subsided along normal faults in the ring fracture zone, and the Plateau Rhyolite (lava flows) poured out across the caldera floor between 600,000 and 500,000 years ago. The faults cut across the central intrusive igneous core of the 50-million-year-old (Eocene) Washburn volcano; the north half of the volcano is still preserved, but the south half subsided as part of the caldera and is now buried by lava flows. (Based on information supplied by H. J. Prostka and R. L. Christiansen.) (Fig. 26)

The final violent eruption 600,000 years ago, although releasing much of the explosive energy of the gases contained in the magma, did not quell all potential volcanic activity in the twin chambers. Molten rock again rose in both of them, and in a few hundreds or thousands of years the overlying caldera floor was domed over the two chambers. One of these prominent domes lies near Old Faithful and the other east of Hayden Valley (figs.22and23D). Soon, too, the magma found its way upward through the wide ring fracture zones encircling the caldera. Pouring out rather quietly from many openings (fig. 23D), the lavas flooded the caldera floor and began to fill the still-smoldering pit. The first lavas appeared soon after the collapse 600,000 years ago, and the latest ones only 60,000-75,000 years ago. The flows were confined chiefly to the caldera proper, but here and there they spilled out across the rim, particularly toward the southwestern part of the Park (fig. 28). Some flows also erupted along fracturesoutside the caldera, the most prominent flow being the very famous one at Obsidian Cliff (fig. 29).

YELLOWSTONE LAKE. View southeast across Yellowstone Lake toward the western foothills and crest of the Absaroka Range. The Absaroka Range is an erosional remnant of a vast pile of volcanic lavas and breccias (Absaroka volcanic rocks) that once covered all of Yellowstone; the lake occupies part of the Yellowstone caldera. (Fig. 27)

The chief rock type in the lava flows is rhyolite, similar in composition to the welded tufts erupted earlier but different in other major characteristics. The rock, for example, shows much contorted layering as evidence of having flowed as a thick liquid across the ground (fig. 30). A coarse brecciated texture is also a common feature, well shown by lavas along the Firehole Canyon drive (fig. 31). Locally, some parts of the flows cooled so rapidly that few crystals formed, and the lava solidified mainly into a natural glass (fig. 32).

RADAR IMAGE of a part of southwestern Yellowstone National Park. The lobate landforms are the edges of a lava flow of the Plateau Rhyolite that forms the Pitchstone Plateau (fig. 1). The low concentric ridges that parallel the toe of the flow are pressure ridges produced by the wrinkling of the nearly solidified crust of lava along the edge of the flow. (Image courtesy of National Aeronautics and Space Administration.) (Fig. 28)

OBSIDIAN CLIFF, Jim Bridger’s famous “mountain of glass.” The rock is rhyolite lava which contains a high proportion of obsidian, a kind of black volcanic glass. Note columnar jointing along the sides of the cliff, similar to that shown by the basalt flows at Tower (fig. 33). The cliff is approximately 200 feet high. (Fig. 29)

THICK RHYOLITE LAVA FLOW along west bank of Firehole River. (Fig. 30)

Closeup view is of a cut surface of rhyolite, showing the striking banding that results from the flowage of viscous molten rock. The dark bands are chiefly concentrations of volcanic glass (also some cavities), and the light bands are concentrations of tiny crystals of feldspar and quartz.

BRECCIATED RHYOLITE LAVA FLOWS along the Firehole Canyon drive. As a lava flow moves outward from its center of eruption, a chilled crust develops along its upper surface and outer edges because of the cooler temperatures in those parts of the flow. Continued movement of the still-molten rock in the interior of the flow causes this crust to break up (brecciate) into angular blocks. The blocks are then tumbled along until the whole mass finally solidifies. (Fig. 31)

OUTCROP OF GLASSY RHYOLITE LAVA along the road between Canyon Village and Norris Junction. The conspicuous lines in the face of the rock outline different layers produced by lava flowage. The feldspar crystals are alined parallel to the direction of flow. (Fig. 32)

In closeup A, dark parts of the rock are volcanic glass (closeup B shows glassy fracture) and light-colored crystals are quartz (blocky) and feldspar (tabular).

Closeup B.

About 30 different flows have been recognized. Grouped within a major rock unit called the Plateau Rhyolite (fig. 5), they cover more than 1,000 square miles. The gently rolling plateau surface of central Yellowstone, broken here and there by clusters of low-lying hills and ridges, is essentially the landscape that characterized the upper surfaces of the lava flows soon after they cooled and solidified. Natural valleys formed between some of the adjacent flows, and in places streams still follow these readymade channels. Rhyolite, in both lava flows and ash-flow tuffs, is by far the predominant rock type seen along the Park roads.

Several basalt flows were erupted along with the more common rhyolite flows, and in the vicinity of Tower Falls they form some of the most unusual rock units in the whole Park area (fig. 33). As the flows cooled, contraction cracks broke the basalt into a series of upright many-sided columns; from a distance they appear as a solid row of fenceposts. They are now covered by younger rocks, but if one could see the upper flat surface of the basalt layers where just the ends of the columns are sticking out, the pattern would be like that seen in a honeycomb.

During the eruptions of the Plateau Rhyolite, at least one relatively small caldera-making event occurred in the central Yellowstone region. This “inner” caldera developed sometime between 125,000 and 200,000 years ago, forming the deep depression now filled by the West Thumb of Yellowstone Lake (fig. 22). Like the main Yellowstone caldera, but on a much smaller scale, it formed as a direct result of the explosive eruption of rhyolitic ash flows and subsequent collapse of an oval-shaped area approximately 4 miles wide and 6 miles long. West Thumb is nearly the same size as Crater Lake, Oregon, which occupies one of the world’s best-known calderas.

With the outpouring of the last lava flows 60,000-75,000 years ago, the forces of Quaternary volcanism finally died down. The hot-water and steam activity, however, still remains as a vivid reminder of Yellowstone’s volcanic past. But who can say even now that we are witnessing the final stage of volcanism? Someday, quite conceivably, there might be yet another outburst of molten rock—only time, of course, will tell.

TWO LEDGES OF BASALT spectacularly exposed in the east wall of the Grand Canyon of the Yellowstone at The Narrows near Tower Falls. The light-colored rocks between the basalt flows are ancient stream gravels deposited about 1½ million years ago, when the channel of the Yellowstone River was farther east and not as deep as it is today. The hill is capped by lake sediments, sand, and gravel deposited when the Yellowstone River was blocked by a glacial dam farther downstream (to the left). The brown rocks at the base of the cliff are Absaroka andesite breccias. (Fig. 33)

Pronounced columnar jointing of the basalt is seen at close range at the edge of the road on the opposite (west) side of the canyon. Inset shows the dense character of the black basalt, which consists of microscopic crystals of feldspar, pyroxene, olivine, and magnetite.

The many episodes of mountain building and volcanism all left their lasting and unmistakable imprints across the face of the Yellowstone country. During the latter part of the Tertiary Period, erosion, too, had begun to make its own deep marks. But only in the last 100,000 years or so have the powerful exterior forces of the earth—chiefly running water and moving ice—had a virtually free hand in shaping the Park’s landscape. Nevertheless, in this short period of time they have wrought profound changes.

A giant boulder of Precambrian gneiss lies among the trees beside the road leading to Inspiration Point on the north rim of the Grand Canyon of the Yellowstone (fig. 34). This boulder, measuring approximately 24×20×18 feet and weighing at least 500 tons, is of considerable interest, not so much for its great size but because it is completely out-of-place in its present surroundings. The boulder rests on rhyolite lava flows of Quaternary age, at least 15 miles from the nearest outcrops of the ancient gneiss to the north and northeast. Obviously, this seemingly immovable chunk ofrock was pushed or carried a long way by some very powerful transporting agent before it was finally dropped. A natural force of such magnitude could only have been exerted by moving ice; in fact, no further proof than this one boulder is needed for us to conclude beyond question that glaciers once existed in Yellowstone. There is, to be sure, much additional evidence that the Park region was extensively glaciated. Deposits of out-of-place boulders (glacial erratics), like the one mentioned above, are found nearly everywhere (fig. 35) and the mountains and high valleys still bear the vivid scars of ice sculpturing (figs.36and37).

GIANT BOULDER (glacial erratic) of Precambrian gneiss near Inspiration Point on the north rim of the Grand Canyon. The boulder, measuring 24×20×18 feet and weighing more than 500 tons, was dropped at this locality by glacial ice; it now rests on the much younger Plateau Rhyolite. The distance that the boulder was carried or pushed was at least 15 miles. (Fig. 34)

The principal requirement for the formation of glaciers is simple: more snow has to accumulate during the winter than is melted during the summer. If this condition continues for a long enough period of time (measured in centuries), the snow compacts to ice, and extensive icefields grow until they finally begin to move under their own weight, thereby becoming glaciers. Records show that the average year-round temperature is 32°-33°F along Yellowstone Lake, 35°F at Old Faithful, and 39°F at Mammoth. Each winter, snow accumulates to depths of 5-10 feet throughout much of the Park. If the average annual temperatures were to decrease a few degrees or the yearly snowfall were to increase a foot or so, either change could possibly herald the beginning of another ice age in the Yellowstone region.

Yellowstone was glaciated at least three times. These glaciations are, from oldest to youngest, the pre-Bull Lake, Bull Lake, and Pinedale. Their precise age and duration are imperfectly known, but estimates based on a few radiometric determinations are: (1) the oldest glaciation (pre-Bull Lake glaciation) began more than 300,000 years ago and ended between 180,000 and 200,000 years ago; (2) Bull Lake Glaciation began about 125,000 years ago and ended more than 45,000 years ago; (3) Pinedale Glaciation began about 25,000 years ago and ended about 8,500 years ago. The pre-Bull Lake and Bull Lake are known only from scattered deposits of rock debris (glacial moraines) and other features, but the distribution of these deposits indicates that glaciers were widespread throughout the region and occurred both between and during eruptions of the Plateau Rhyolite. The effects of the Pinedale glaciers, on the other hand, are obvious in many parts of the Park, and the history of this youngest glacial cycle (described below) is known in much greater detail than that of the two older ones.

In the early stages of Pinedale Glaciation, an enormous icefield built up in the high Absaroka Range southeast of the Park area. A glacier, fed by this icefield, flowed northwarddown the upper Yellowstone valley and into the basin now occupied by Yellowstone Lake. At about the same time, another great icefield formed in the mountains north of the Park and sent long tongues of ice southward toward the lower Yellowstone and Lamar River valleys. Smaller valley glaciers flowed westward out of the Absaroka Range along the east edge of the Park, and still others formed along the main ridges and valleys of the Gallatin Range, in the northwestern part of the Park. Thus, many huge masses of ice from the north, east, and southeast converged and met in the Park. At this stage, probably about 15,000 years ago, only the west edge of the Park, and perhaps a few of the highest peaks and ridges within the Park, remained free of ice. It is interesting to note that although ice moved across and buried the ancestral Grand Canyon of the Yellowstone, it did not flow down and scour the canyon (fig. 36). If it had, the canyon would look much different than it does today (fig. 41).

GLACIATED TERRAIN along the Northeast Entrance road. The boulders, many of them measuring 10 feet across or more, were carried into the area by ice flowing down Slough Creek from mountains north of the Park during the Pinedale Glaciation. As the glaciers melted, the boulders were left stranded in hummocky, morainal deposits. Shallow depressions in the irregular topography are now commonly filled by small ponds. (Fig. 35)

For the next 10,000 years, the ice thickened and spread out over more and more of the Park area. The mass centered over the Yellowstone Lake basin grew to a depth of 3,000 feet or more and dominated the entire scene; it formed a broad “mountain” of ice which became so high that it caused more snow to fall upon itself and was cold enough to prevent much of this snow from melting. Eventually the Pinedale glaciers covered about 90 percent of Yellowstone (fig. 38).

CANYON PROFILES. Typical profiles of a canyon cut by a stream (A) and of a canyon gouged by a glacier (B). Glacial cirques (C) are shown at the head and high on the side of the glaciated valley. (Fig. 36)

GLACIAL CIRQUE on east face of Electric Peak, northern Gallatin Range. During several episodes of glaciation, this steep-walled amphitheaterlike valley was cut and filled by ice which fed glaciers moving downslope to the lower right. The cirque floor is now covered by a thick deposit of rock rubble underlain in part by ice, and the whole mass is still moving slowly downhill as a rock glacier. The dark rock at lower right is part of the Electric Peak stock, composed of diorite (fig. 20) and other kinds of intrusive igneous rocks. The rocks in the cirque walls are chiefly Cretaceous shales (light to moderately dark color) with thin sills of igneous rock (very dark color). (West-looking oblique aerial photograph, courtesy of William B. Hall, University of Idaho.) (Fig. 37)

After their maximum advance, the Pinedale glaciers began to melt, leaving behind the rock debris they had gouged from the landscape and had pushed or carried along with them. These glacial moraines are now found in many areas throughout the Park. In places, glacial ice and (or) rock debris formed natural dams across stream valleys, thereby impounding lakes. Parts of Hayden Valley, for example, contain layers of very fine sand, silt, and clay several tens of feet thick (fig. 39) that accumulated along the bottom of a large lake. This lake formed behind a glacial dam across the Yellowstone River near Upper Falls. Some of the glacial dams broke and released water catastrophically, causing giant floods; the occurrence of one such flood is particularly evident along the Yellowstone River valley near Gardiner, Montana.

By about 12,000 years ago the thick Pinedale ice sheet had melted entirely from the Yellowstone Lake basin and most other areas of the Park, although valley glaciers continued to exist in the mountains until about 8,500 years ago. Then, following a short period of total disappearance, small icefields formed again in the heads of some of the higher mountain valleys. Since the melting of the Pinedale ice, however, none has descended as a glacier into the lower stretches of the valleys. Even though a few snowfields persist locally throughout the summers (except during the warmest years), no glaciers exist in the Park at the present time.

EXTENT OF ICE in Yellowstone National Park during the maximum spreading of the Pinedale glaciers, probably about 15,000 years ago. Long arrows indicate direction of strong flowage of ice; short arrows show direction of less vigorous ice flowage. The dark-blue area shows the main ice mass centered over the Yellowstone Lake basin in the southeast corner of the Park. Many of the high peaks and ridges such as Mount Washburn, which are here shown free of ice, were glaciated at least once during the past 250,000 years. Whether they were covered by the Pinedale glaciers, however, is still an unresolved question. (Based on information supplied by G. M. Richmond, K. L. Pierce, and H. A. Waldrop.) (Fig. 38)

FLAT-LYING BEDS of fine sand, silt, and clay near the mouth of Trout Creek in Hayden Valley. These beds were deposited in a glacially dammed lake that covered part of Hayden Valley when the Pinedale glaciers were melting. The height of the streambank is about 40 feet. (Fig. 39)

Douglas-fir branch and cones.

WATERFALLS in Yellowstone National Park. (Fig. 40)A, Lewis Falls on the Lewis River. The falls cascade over the steep edge of a rhyolite lava flow.

WATERFALLS in Yellowstone National Park. (Fig. 40)

A, Lewis Falls on the Lewis River. The falls cascade over the steep edge of a rhyolite lava flow.

B, Upper Falls on the Yellowstone River. The brink of the falls marks the contact between dense, resistant rhyolite lava (which forms the massive cliff) and more easily eroded rhyolite lava containing a high proportion of volcanic glass immediately downstream, as shown infigure 42.

C, Gibbon Falls on the Gibbon River. The river tumbles over a scarp etched in the Yellowstone Tuff. The scarp first formed along faults at the north edge of the Yellowstone caldera 600,000 years ago, at a point that now lies ¼ to ½ mile downstream. Continued erosion has caused the falls to recede northward to their present position.

D, Tower Falls on Tower Creek. The rocks at the brink of the falls, and in the vertical cliff beneath, are coarse breccias and conglomerates of the Absaroka volcanic rocks. The channel of Tower Creek has not been cut down rapidly enough to keep pace with the downcutting of the main channel of the Yellowstone River, which lies a short distance downstream from the base of the falls.

Yellowstone is, among its many attributes, the source of large and mighty rivers. Located across the Continental Divide, the Park feeds two of the most extensive drainage systems in the nation—(1) the Missouri River system (and ultimately the Mississippi River) on the Atlantic side, via the Yellowstone, Madison, and Gallatin Rivers, and (2) the Columbia River system on the Pacific side, via the Snake River (fig. 1). These streams are fed by an annual precipitation which averages about 17 inches at Old Faithful and Mammoth, but which is considerably greater in the mountain ranges.

Many stretches of the main river valleys in Yellowstone are broad and flat bottomed. In these, the stream gradients range from about 10 to 30 feet per mile, and there is little erosion going on at present (Hayden Valley is a good example,fig. 4). But here and there the gradients are steeper, and the valleys are narrow and rugged. In some places these streams drop 50 or even 100 feet per mile, and the fast-moving waters have carved deep V-shaped gorges (fig. 36).

Waterfalls, features for which Yellowstone is also justly famous (fig. 40), generally result from abrupt differences in rock hardness. If a stream flows over rocks that are more resistant to erosion than the rocks immediately downstream, a ledge or bench will form across the streambed at that placebecause the less resistant rocks are worn away faster. And, as the ledge becomes higher, the softer downstream rocks will erode even faster. A true waterfall is one in which there is a free, vertical fall of water. If the ledge or ledges form only a rough, steep runway in the streambed, then the term “rapids” or “cascades” is more appropriate.

The existence of many waterfalls in Yellowstone today is due in large part to the fact that, because of recent volcanism and glaciation, much of the region’s topography is very young in terms of geologic time. Streams, even some of the largest ones, have not had enough time to wear away all the features that may produce waterfalls, cascades, or rapids along their channels. This is particularly true along the margins of lava flows, where there are sharp dropoffs between the tops of the flows and the lower ground beyond. The Grand Canyon of the Yellowstone and the Upper and Lower Falls, well illustrate the erosive power of running water.

Except for Old Faithful, the Grand Canyon of the Yellowstone is probably the best known and most talked about and photographed feature in the Park (fig. 41). Although not so deep or wide as some of the other great canyons in America, its sheer ruggedness and beauty are breathtaking. Here the aptness of the name “Yellowstone” can be fully appreciated and understood, for the viewer is at once engulfed in a sea of yellow hues streaked and tinted with various shades of red and brown.

GRAND CANYON AND LOWER FALLS of the Yellowstone River, as viewed upstream (southwest) from Artists Point on the south rim. The yellow-hued rocks lining the canyon walls are soft, hydrothermally altered rhyolite lavas. The rocks at the brink of the falls consist of less altered and therefore more resistant rhyolites. The falls, 309 feet high, formed at the contact between the hard and soft rhyolite units. (Photograph courtesy of Sgt. James E. Jensen, U.S. Air Force.) (Fig. 41)

DEVELOPMENT OF GRAND CANYON. Profiles along the floor of the Grand Canyon of the Yellowstone as it appears today (C) and as it appeared at two older stages in its development (A and B). Note particularly the various kinds of rocks through which the canyon has been cut, and how rock differences have influenced the location of the two falls. Diagonal lines indicate unaltered rhyolite; large dots, rhyolite with much volcanic glass; small dots, hydrothermally altered rhyolite; and circles and dots, Absaroka volcanic rocks. (Based on information furnished by R. L. Christiansen and G. M. Richmond; vertical scale is exaggerated about 10 times.) (Fig. 42)

At first glance, the canyon may appear to be a giant crack which suddenly opened up and into which the Yellowstone River then plunged headlong over high waterfalls at its southwest end. This, of course, is not the way the canyon formed. Nevertheless, it is apparent that certain unusual conditions caused the river, after winding slowly through flat-floored Hayden Valley for about 13 miles, to cut a precipitous gorge 1,000-1,500 feet deep and 20 miles long (fig. 42C). A full explanation must be based on all the many events surrounding the eruption of the Yellowstone Tuff, the collapse of the Yellowstone caldera, the outpouring of the Plateau Rhyolite, and the various episodes of glaciation. Geologic studies show that all these events took place while the canyon was being cut, and that each one played an important role in its development. Hot-water and steam activity likewise was a significant factor. However, despite its many complexities, the history of the Grand Canyon can be divided into a few major stages, as outlined below:

1. From more than 2,000,000 years ago to about 600,000 years ago, a shallow canyon was gradually being cut into the Absaroka volcanic sequence by the ancestral Yellowstone River as it eroded headward from a point near the present confluence of the Yellowstone and Lamar Rivers (fig. 33). By the time of the climactic volcanic eruption in central Yellowstone 600,000 years ago, the head of the “old” canyon probably had been eroded southward nearly to the place where the north rim of the Yellowstone caldera was to form later (fig. 42A). This point now lies about 5 miles below Lower Falls.2. Ash-flow tuffs that were erupted 600,000 years ago filled the “old” canyon, and the river recarved its channel, chiefly along its previous course.3. A large lake formed behind (south of) the north rim of the caldera, the damming resulting in part from lava flows of Plateau Rhyolite that poured out across the caldera floor in this area between 600,000 and 500,000 years ago. Eventually the lake rose and spilled northward into the head of the “old” canyon, causing additional downcutting in what is now the lower 15-mile stretch of the canyon.4. As the lake emptied, the river began to erode upstream into the thick rhyolite lava flows toward the present site of Lower Falls; the process was very similar to that of a common stream gully eroding headward into a hillside. At a stage somewhat more than 300,000 years ago, the head of the canyon probably lay near the falls, and the river had cut a channel 400-600 feet deep along this upper 5-mile stretch (fig. 42B).5. Approximately 300,000 years ago the canyon area was covered by ice during pre-Bull Lake glaciation. During and after the retreat of this ice, sediments accumulated in a lake that occupied the upper reaches of the canyon between the present site of Upper Falls and Inspiration Point. Subsequently, very little downcutting was accomplished until about 150,000-125,000 years ago, when the canyon was eroded nearly to its present depth.6. Canyon development was further interrupted by the advance and retreat of glaciers during Bull Lake and Pinedale Glaciations. During and since the melting of the Pinedale glaciers about 12,000 years ago, the canyon has attained its present depth, and its walls have acquired much of their picturesque erosional form. The Yellowstone River now maintains a fairly uniform gradient (60-80 feet per mile) throughout the 20-mile-long gorge, even though different segments of the canyon were cut at different times and through different kinds of rocks (fig. 42C).

2. Ash-flow tuffs that were erupted 600,000 years ago filled the “old” canyon, and the river recarved its channel, chiefly along its previous course.

3. A large lake formed behind (south of) the north rim of the caldera, the damming resulting in part from lava flows of Plateau Rhyolite that poured out across the caldera floor in this area between 600,000 and 500,000 years ago. Eventually the lake rose and spilled northward into the head of the “old” canyon, causing additional downcutting in what is now the lower 15-mile stretch of the canyon.

4. As the lake emptied, the river began to erode upstream into the thick rhyolite lava flows toward the present site of Lower Falls; the process was very similar to that of a common stream gully eroding headward into a hillside. At a stage somewhat more than 300,000 years ago, the head of the canyon probably lay near the falls, and the river had cut a channel 400-600 feet deep along this upper 5-mile stretch (fig. 42B).

5. Approximately 300,000 years ago the canyon area was covered by ice during pre-Bull Lake glaciation. During and after the retreat of this ice, sediments accumulated in a lake that occupied the upper reaches of the canyon between the present site of Upper Falls and Inspiration Point. Subsequently, very little downcutting was accomplished until about 150,000-125,000 years ago, when the canyon was eroded nearly to its present depth.

6. Canyon development was further interrupted by the advance and retreat of glaciers during Bull Lake and Pinedale Glaciations. During and since the melting of the Pinedale glaciers about 12,000 years ago, the canyon has attained its present depth, and its walls have acquired much of their picturesque erosional form. The Yellowstone River now maintains a fairly uniform gradient (60-80 feet per mile) throughout the 20-mile-long gorge, even though different segments of the canyon were cut at different times and through different kinds of rocks (fig. 42C).

The spectacular erosional development in the upper 5-mile segment of the Grand Canyon, which is the only part seen by most Park visitors, except for the very lower end near Tower Falls (fig. 33), has taken place mostly within the past 150,000-125,000 years. One reason for such a rapid rate of erosion stems from the fact that this part of the canyon overlies one of the wide ring fracture zones of the Yellowstone caldera (fig. 22). The fracture zone extends to great depth, providing a ready avenue of travel for the upflow of hot water and steam rising in the Yellowstone thermal system, as described in the following chapter. Through many thousands of years, the upward percolation of the hot fluids has caused severe chemical and physical changes (known ashydrothermal alteration) in the rhyolite lava flows. One spectacular result of the alteration has been the change from the normal brown and gray color of the rhyolites to the bright yellow and other colorful hues now seen in the canyon walls (as well as in many other places throughout the Park). Another significant result of alteration has been the weakening of the rocks; that is, the altered rocks are softer and less resistant to erosion than unaltered rocks. Hence, the river has beenable to erode these softer rocks, upstream to Lower Falls, at a very rapid rate.

The position of Lower Falls, as might be expected, coincides with a change from highly altered to less altered rhyolite; the difference in the erosion rates of the two kinds of rocks here is self-evident (figs.41and42C). The position of Upper Falls is likewise closely controlled by differences in rock hardnesses. The rhyolites on the upstream side are hard and dense, whereas those on the downstream side contain a high proportion of volcanic glass which causes them to be more easily eroded (fig. 42C).

Swan family.

COMMON KINDS OF THERMAL FEATURES in Yellowstone National Park. (Fig. 43)A, Hot springs and terraces colored by algae at Mammoth Hot Springs.

COMMON KINDS OF THERMAL FEATURES in Yellowstone National Park. (Fig. 43)

A, Hot springs and terraces colored by algae at Mammoth Hot Springs.

B, Castle Geyser erupting in Upper Geyser Basin.

C, Fountain Paint Pots in Lower Geyser Basin.

D, pool in Lower Geyser Basin.

Although Yellowstone is geologically outstanding in many ways, the great abundance, diversity, and spectacular nature of its thermal (hot-water and steam) features were undoubtedly the primary reasons for its being set aside as our first National Park (fig. 43). The unusual concentration of geysers, hot springs, mudpots, and fumaroles provides that special drawing card which has, for the past century, made the Park one of the world’s foremost natural attractions.

To count all the individual thermal features in Yellowstone would be virtually impossible. Various estimates range from 2,500 to 10,000, depending on how many of the smaller features are included. They are scattered through many regions of the Park, but most are clustered in a few areas called geyser basins, where there are continuous displays of intense thermal activity. (Seefrontispiece.) The “steam” that can be seen in thermal areas is actually fog or water droplets condensed from steam; so the appearance of individual geyser basins depends largely on air temperature and humidity. On a warm, dry summer day, for example, the activity may seem very weak (fig. 44), except where individual geysers are erupting. On cold or very humid days, however, “steam” plumes are seen rising from every quarter.

An essential ingredient for thermal activity is heat. A body of buried molten rock, such as the one that produced volcanic eruptions in Yellowstone as late as 60,000 to 75,000 years ago, takes a long time to cool. During cooling, tremendous quantities of heat are transmitted by conduction into the solid rocks surrounding the magma chamber (fig. 45). Eventually the whole region becomes much hotter than non-volcanic areas (fig. 46). Normally, rock temperatures increase about 1°F per 100 feet of depth in the earth’s crust, but in the thermally active areas of Yellowstone the rate of temperature increase is much greater. The amount of heat given off by the Upper Geyser Basin, for example, is 800 times the amount given off by normal (nonthermal) areas of the same size. This excess heat is enough to melt 1½ tons of ice per second! And, contrary to popular opinion, the underground temperatures have not cooled measurably in the 100 years that records have been kept on the thermal activity in the Park. In fact, geologic studies indicate that very high heat flows have continued for at least the past 40,000 years.

NORRIS GEYSER BASIN, as viewed northward from the Norris Museum. This is one of the most active thermal areas in Yellowstone, but the photograph was taken on a warm dry summer day when little hot-water and steam activity was visible from a distance. Clouds of water droplets (the visible “steam” in thermal areas) normally form only when the air is cool and (or) moist. The floor of the basin is covered by a nearly solid layer of hot-spring deposits. (Fig. 44)

HEAT FLOW AND SURFACE WATER. Diagram showing a thermal system, according to the explanation that water of surface origin circulates and is heated at great depths. (Based on information supplied by D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H. Truesdell.) (Fig. 45)

INFRARED IMAGE of a part of Upper Geyser Basin. Infrared instruments, sensitive to heat, are able to detect “hot” spots in the landscape. Note especially the sharp “image” of Old Faithful. (Image courtesy of National Aeronautics and Space Administration.) (Fig. 46)

A second, equally essential ingredient for thermal activity is water. Many thousands of gallons are discharged by the hot springs and geysers in Yellowstone every minute—where does all this water come from? Studies show that nearly all the water originates above ground as rain or snow (meteoric water;fig. 45), and that very little comes from the underlying magma (magmatic water).

The mechanism for heating the water, on the other hand, is a matter of some uncertainty. Until a few years ago theheating was assumed to occur near the ground surface and to be caused by hot magmatic gases (mostly steam) rising from the underlying magma chamber. Deep wells drilled recently in many thermal areas throughout the world (including research drill holes in Yellowstone), however, suggest a better explanation. According to this explanation, the surface water enters underground passages (fractures and faults) and circulates to great depths—as much as 5,000-10,000 feet in some areas (fig. 45)—there to become heated far above its surface boiling point. Research drill holes in Yellowstone, for example, have demonstrated that water of surface origin exists at all depths at least to the maximum drilled (1,088 feet), and that the water reaches temperatures up to at least 465°F. The increase in temperature with depth causes a corresponding decrease in the weight (density) of the water. Because of this, the hot, “lighter,” water begins to rise again toward the ground surface, pushed upward by the colder, “heavier,” near-surface water which sinks to keep the water channels filled. Thus is set into motion a giantconvection currentwhich operates continuously to supply very hot water to the thermal areas (fig. 45). Just how deep the waters circulate in Yellowstone no one really knows; as a guess, the depth probably is at least 1 or 2 miles.

The effect of pressure on the boiling temperature of water also plays a vital role in thermal activity. In a body of water, the pressure at the surface is that exerted by the weight of air above it (atmospheric pressure). Water under these conditions boils at 212°F at sea level and at about 199°F at the elevation of most of the geyser basins in Yellowstone. However, water at depth not only is subjected to atmospheric pressure but also bears the added weight of the overlying water. Under such additional pressures, water boils only when the temperature is raised above its surface boiling point. In a well 100 feet deep at sea level, for example, the water at the bottom would have to be heated to 288°F before it will boil. Thus it follows that in the underground “workings” of hot springs or geysers, (1) The deepest water is subjected to the greatest pressures, and (2) these deeper waters (in Yellowstone) must be heated well above 199°F before they can actually begin to boil. By this same reasoning but in reverse, if the pressure is released, which happens as the waterrises toward the ground surface, the “hotter-than-boiling” water will then begin to boil. The boiling will be rather quiet if the pressure is released gradually, as in most hot springs. But if the pressure is released suddenly, boiling may become so violent that much of the water flashes explosively into steam, expanding to several hundred times its normal volume. This expansion provides the necessary energy for geyser eruptions.

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